Taxability of SIPPs

What is the latest (2017) consensus about the US tax liability of SIPP accounts?

When I last looked into this in 2013, there was a strong disagreement among tax professionals about how to report SIPPs. The more conservative stance being that SIPPs are not qualified retirement vehicles in the eyes of the IRS and therefore must be reported as a foreign grantor trust, with all the underlying investments reported as PFICs. The more liberal interpretation was that SIPPs are qualified and you can apply the US/UK tax treaty to state your position and claim exemption.

This page contains some details from tax professionals: http://web.archive.org/web/20120117195157/http://www.taxalmanac.org/index.php/Discussion:US_taxation_of_UK_Stakeholder_Pension_/_SIPP

But as you can see, that page is years old and no longer exists outside of the internet archive.

So, while I don't doubt that this is still a grey area and open to interpretation, I'm wondering if there has been any new information come to light in recent years that has caused the community to start leaning more in one direction or the other on this issue?

Here's what my tax attorney has advised. It's still subjective, but here's a stance we have taken. My SIPP is considered a pension for purposes of the treaty as more than 50% of the contributions were made by a company, and not personal contributions. Therefore, it's not considered a foreign grantor trust and no PFIC reporting. I report the profits on the SIPP on my 8833 so as not to pay tax on it until withdrawal. I've done this for my last 4 tax returns with no issues.

I have just started taking periodic payments and claim 25% tax free as it's not a lump sum. It's also my and my attorney's opinion that as I didn't exclude the contributions from income, then I have a "basis" in the plan and pay tax only on the income generated by the SIPP, and not when withdrawing the capital. I have just started another thread on this as I have a question for the experts RE this.

That's not quite how basis works using the general rule in IRC Section 72. You'd need to figure how the exclusion ratio.  Hopefully you have done this...